The present invention generally relates to an apparatus and method for a rich, quick mix combustion system that provides low levels of NOx, carbon monoxide, unburned hydrocarbons, and smoke. More specifically, the present invention relates to an apparatus and method for a rich, quick mix combustion system comprising a premixing chamber located upstream of a combustion chamber.
Gas turbine engines, such as those which may be used to power modern commercial aircraft, may include a compressor for pressurizing a supply of air, a combustor for burning a hydrocarbon fuel in the presence of the pressurized air, and a turbine for extracting energy from the resultant combustion gases. The combustor may include radially spaced apart inner and outer liners. The liners may define an annular combustion chamber that resides axially between the compressor and the turbine. Arrays of circumferentially distributed combustion air holes may penetrate each liner at multiple axial locations to admit combustion air into the combustion chamber. Fuel may be supplied to the combustion chamber by one or more fuel nozzles.
Combustion of the hydrocarbon fuel may produce a number of reaction products including oxides of nitrogen (NOx). NOx emissions are the subject of increasingly stringent controls by regulatory authorities. Accordingly, engine manufacturers strive to minimize NOx emissions.
A principal strategy for minimizing NOx emissions is referred to as a rich burn, quick quench, lean burn (RQL) combustion system. The RQL strategy recognizes that the conditions for NOx formation are most favorable at elevated combustion flame temperatures, i.e., when the fuel-air ratio is at or near a stoichiometric ratio. A combustor configured for RQL combustion may include three serially arranged combustion zones: a rich burn zone at the forward end of the combustor, a quench or dilution zone axially aft of the rich burn zone, and a lean burn zone axially aft of the quench zone.
During engine operation, a portion of the pressurized air discharged from the compressor may enter the rich burn zone of the combustion chamber. Concurrently, the fuel nozzle may introduce a stoichiometrically excessive quantity of fuel into the rich burn zone. The resulting stoichiometrically rich fuel-air mixture may be ignited and burned to partially release the energy content of the fuel. The fuel rich character of the mixture may inhibit NOx formation in the rich burn zone by suppressing the combustion flame temperature. This condition may also resist blowout of the combustion flame during any abrupt reduction in engine power.
The fuel rich combustion products generated in the rich burn zone then enter the quench zone where the combustion process continues. Jets of pressurized air from the compressor may enter the combustion chamber radially through combustion air holes. The air mixes with the combustion products entering the quench zone to support further combustion and release additional energy from the fuel. The air may also progressively consume fuel in the fuel rich combustion products as they flow axially through the quench zone and mix with the air to produce a lean combustion product. Initially, the fuel-air ratio of the combustion products may change from fuel rich to stoichiometric, which may cause an attendant rise in the combustion flame temperature. Since the quantity of NOx produced in a given time interval increases exponentially with flame temperature, substantial quantities of NOx can be produced during the initial quench process. As the quenching continues, the fuel-air ratio of the combustion products changes from stoichiometric to fuel lean, causing an attendant reduction in the flame temperature. However, until the mixture is diluted to a fuel-air ratio substantially lower than stoichiometric, the flame temperature remains high enough to generate considerable quantities of NOx.
Finally, the lean combustion products from the quench zone flow axially into the lean burn zone where the combustion process concludes. Additional jets of compressor discharge air may be admitted radially into the lean burn zone. The additional air supports ongoing combustion to release energy from the fuel and regulates the peak temperature and spatial temperature profile of the combustion products. Regulation of the peak temperature and temperature profile may also protect the turbine from exposure to excessive temperatures and excessive temperature gradients.
Because most of the NOx emissions originate during the quenching process, it may be beneficial for the quenching to progress rapidly, thus limiting the time available for NOx formation. It may also be beneficial for the fuel and air to become intimately intermixed, prior to, and throughout the combustion process, otherwise, even though the mixture flowing through the combustor may result in combustion products that may be stoichiometrically lean overall, the combustion products may include localized pockets where the fuel-air ratio is stoichiometrically rich. Because of the elevated fuel-air ratio, fuel rich pockets may burn hotter than the rest of the mixture, thereby promoting additional NOx formation and generating local “hot spots” or “hot streaks” that may damage the turbine.
Attempts directed to lowering NOx emissions in gas turbine exhaust include U.S. Pat. No. 6,606,861 to Snyder (Snyder), which is directed to a combustor for a gas turbine engine, which includes inner and outer liners with a row of dilution air holes penetrating through each liner. The row of holes in the outer liner comprise at least a set of large size, major outer holes and may also include a set of smaller size minor outer holes circumferentially intermediate neighboring pairs of the major outer holes. The row of holes in the inner liner include dilution air holes circumferentially offset from the major outer holes and may also include a set of minor holes circumferentially intermediate major inner holes. The major and minor holes admit respective major and minor jets of dilution air into the combustor. The distribution of major and minor holes and the corresponding major and minor dilution air jets helps to minimize NOx emissions and regulates the spatial temperature profile of the exhaust gases discharged from the combustor. The fuel nozzle (referred to in Snyder as a fuel injector) injects fuel directly into the combustion chamber. Each of the liners in Snyder includes a support shell, a forward heat shield, and an aft heat shield. Snyder may thus result in a complicated arrangement, wherein the heat shields may be cooled using film cooling holes that penetrate through each heat shield, and each shell may be cooled using impingement cooling holes that penetrate through each shell.
Another attempt directed to lowering NOx emissions in gas turbine exhaust includes U.S. Pat. No. 6,286,300 to Zelina et al. (Zelina), which is directed to an annular combustor having fuel preparation chambers mounted in the dome of the combustor. In Zelina, the fuel preparation chamber is defined within a wall disposed about a center axis, which extends from an inlet end of the fuel preparation chamber to an outlet end of the fuel preparation chamber longitudinally along the center axis. An air swirler and a fuel atomizer are mounted to an inlet plate attached to the inlet end of the fuel preparation chamber. The air swirler provides swirled air to the fuel preparation chamber, while the atomizer provides a fuel spray to the fuel preparation chamber. Downstream of the inlet end of the fuel preparation chamber is an outlet end having an inwardly extending conical wall, referred to in Zelina as a chimney. This chimney restricts flow out of the fuel preparation chamber, thus the chimney acts to compress the mixture of fuel and air as it exits the fuel preparation chamber at the outlet end.
Zelina is thus directed to a design involving a separate swirler being mounted to the inlet of the fuel preparation chamber. Zelina also requires a conical chimney wherein the fuel/air mixture must first be compressed prior to ignition of the fuel/air mixture. As can be seen, there is a need for providing a thoroughly mixed fuel and air mixture to a combustion chamber of a gas turbine utilizing a simple design, without adversely affecting or compromising engine performance.